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August 24, 2011

THE SHORT VERSION: By popular demand, here’s an “executive summary” and Q&A for the Objective2 (O2) headphone amp. There are links sprinkled throughout leading to revised information in the previous three large articles and elsewhere.

QUESTIONS ANSWERED: Before you post a comment to one of the articles, forum threads, etc., please see if your question has already been answered below.

WHAT’S AN O2? The Objective2 (O2) is my attempt at a “One-Size-Fits-Nearly-All” headphone amp. It can drive most any headphone from the most sensitive IEMs to some really power hungry full size cans from 16 to 600 ohms. It’s small, inexpensive, and runs on batteries or AC power. The design is freely available to everyone subject to the License terms. See: O2 Headphone Amp

WORRIED YOU’RE NOT GETTING THE BEST SOUND? Reading through the forums it’s obvious a huge number of headphone lovers are worried they’re not getting the best sound from their headphones. Their worry may be justified as most of the headphone amps (and headphone DACs) on the market have one or more significant limitations or problems. Many only work well with a limited range of headphones. See: Going Shopping

LESS WORRY: Very few manufactures provide anything resembling credible specifications for their headphone amps and very few are independently tested on proper equipment. So you rarely know what you’re getting. I’ve discovered significant problems or limitations with everything I’ve tested with a headphone jack on it. The O2 is designed to ease such concerns by working well with nearly all headphones and having fully documented performance. See: O2 Headphone Amp

WHY DID YOU CALL IT THE OBJECTIVE2? I’m an electrical engineer, serious audio geek, and took a very objective approach to the design. It’s a minimalist amp focused on the best performance possible for the least amount of money. See: Objective Pitch

POWER TO SPARE: In the testing I’ve done, with more than a dozen different headphones, either my ears, or the headphones, reach their limits long before the O2 does. I suspect with some worst case headphones (like some of the planars from HIFiMan) the O2 will merely get genuinely loud at full output. But for more typical headphones, the O2 will be well under its full output. Compared to most battery powered amps that are often pushed to the edge of, or well into, clipping that’s a very audible difference. The O2 is loafing when most portable amps are straining. And, unlike dual battery Cmoy amps, the O2 not only has lots of voltage, but it also has lots of current for low impedance loads that require it—like AKG K701s, Audeze LCD-2s and HiFiMan planars. And the O2’s two stage design vastly improves noise and distortion over a single stage Cmoy.

HEADPHONE SAFETY: Unlike every two battery Cmoy or similar design I know of, including the $650 Emmeline SR-71 Blackbird, the O2 won’t try to destroy your headphones if you run the batteries low or one battery becomes disconnected. It has a power management circuit to shut it down if there’s anything wrong with the power preventing dangerous levels of DC at the output.

LOW OUTPUT IMPEDANCE = TIGHT BASS: If you’ve been using a headphone amp or source with a significant output impedance you may be in for a treat with the O2. A sufficiently low output impedance provides improved bass damping with many headphones and the most accurate frequency response instead of an exaggerated peak at resonance. With my Beyer DT770-Pro 80 headphones, for example, the difference in the bass performance is easily noticeable (and survives a blind test) when using a high output impedance source versus the O2. With the O2 the bass is tighter and more controlled. Lots of headphone source gear has a high enough high output impedance to cause audible problem. The O2 solves these problems. See: Amplifier Impedance

WHAT DOES IT COST? The O2 can be built in functional “bare board” form for $30 - $40. The total parts cost with everything, including a pre-made front panel, batteries, enclosure, and AC adapter is around $75 plus shipping. The DIYer can have a complete amp for $80 – $100 all inclusive. Even fully assembled complete with batteries and AC power adapter it can still be under $150. See: O2 Price Details.

WILL THE O2 DRIVE MY HEADPHONES? Short of a few discontinued models, like AKG K1000 “earspeakers”, the answer is very likely yes. The O2 can drive normally challenging headphones including the AKG K701 and even HiFiMan planars. See: More Power?

DOES THE O2 HAVE ANY HISS WITH SENSITIVE IEMs? The O2 is the first amp I’ve tested that’s dead silent with any headphones I know of at any volume setting, and any applicable gain, including with my ultra sensitive UE SuperFi Pros. The noise is –116 dBv or –133 dB referenced to full output.

WILL YOU BUILD ONE FOR ME? There are already individuals and businesses offering to build the O2 for others either as a completed circuit board or a fully assembled amp. See O2 Resources for various options. I’m staying out of the commercial aspect. I expect as the O2 becomes more established there will be additional purchasing options in the future and I’ll be updating the O2 Resources links.

HOW CAN I GET ONE? Right now the O2 is mainly a DIY project and there will likely be more options in the near future. For what’s available today, see O2 Resources.

WHY DID YOU DESIGN IT? I shopped around for a decent headphone amp that didn’t have significant problems or limitations and couldn’t find any that were reasonably priced. Sadly, I mostly found heavily flawed designs unable to deliver what the recording engineer intended. I also wanted to show what can be done by properly implementing about $30 worth of parts. See: O2 Motivation

DO YOU HAVE A COMMERCIAL INTEREST IN THE O2? No.I don’t get any money, revenue or income of any kind from the O2. It’s open source hardware and I’m not selling anything. I don’t have a commercial interest in anything audio related at the moment. My day job is in another field.

WHY ARE YOU GIVING IT AWAY? I’m mainly trying to prove proper design goes a lot further than using expensive designer parts, following audiophile myths, trying to re-invent the wheel, resurrecting ancient single-ended designs, etc. The idea is to raise the bar in the industry as a whole. There are just too many marginal and/or overpriced headphone amps—both DIY and commercial. See: O2 Motivation

WHERE ARE THE MEASUREMENTS? The O2’s performance has been thoroughly documented using industry standard measurement practices. For a summary see: O2 Review. For the full measurements see: O2 Measurements

WHAT ABOUT THE SOUND QUALITY? The O2 compares very favorably in listening tests against the critically acclaimed Benchmark DAC1 Pre’s headphone output. An O2 user has compared it to a $1000+ AMB beta22 (b22) and wasn’t sure he could hear any difference. As more amps get evaluated more and more favorable reviews are coming in. I have encouraged blind listening tests against other gear as well. See: O2 Subjective Pitch

COMPETITIVE CHALLENGES: Is the Camaro faster than the Mustang? Let’s take them both to the track and find out. First across the finish line wins! I’ve challenged others to beat the O2’s objective measured performance with amps up to three times the assembled price. And for the subjectivists, I’m willing to put the O2 up against suitable amps of any price in a proper blind listening test. See An Open Challenge for the details.

DOES THE O2 HAVE ADJUSTABLE GAIN? The O2 has a gain switch on the front panel. You might use one setting with IEMs and the other with power hungry full size cans. Or one gain for portable use and the other for home use. The standard gains are 2.5X and 6.5X but can be internally changed to anything from 1X to 12X. See: Gain Settings and All About Gain.

HOW LONG IS THE BATTERY LIFE? The regular version runs for about 8 hours and, by swapping out 3 ICs, it increases to about 25 hours. See: O2 Low Power Option

ARE THERE FORUM DISCUSSIONS? Yes, you can find the links in the O2 Resources section.

CAN I UPGRADE THE OP AMPS? You could, but it would be a waste of money and might degrade the performance. I tested nearly two dozen op amps and the O2 is optimized for the op amps that proved best. See: Op Amp Myths and Op Amp Measurements. I have offered $500 for charity to anyone who can hear differences between op amps that measure reasonably well. Also check out: Designer Components.

WHAT ABOUT CLAIMS ON HEAD-FI THE O2 IS FLAWED? As far as I know, the criticism was started by those with financial interests in more expensive products. And it’s mostly been put to rest. Please consider where the O2’s critics obtain their income (selling parts, amps, ads to sponsors, etc.) and/or their personal investment in far more expensive gear. I’ve requested the critics participate in either of the two official O2 forum threads on ABI and diyAudio but it’s telling they have refused. Instead, they seem to be hiding behind the admins who banned me at Head-Fi. I welcome an open discussion on either of the official O2 threads, valid criticisms, proper blind comparisons, and professional measurements. See Competitive Challenges above. The O2 isn’t perfect, but when you consider its size, price, battery operation, etc. I believe it’s hard to beat and any criticisms should be kept in proper perspective for a portable amp that costs $25 – $150 and can drive most any headphone.

DOES THE INPUT OVERLOAD? The O2 will not overload in any sort of normal use. Here are the facts (all input voltages are Vrms):

On AC Power - At 2.5X gain the O2 can handle 2.8V and at 6.5X gain it can handle over 1V.

On Battery Power – At 2.5X gain the O2 can handle 1.8V and at 6.5X it can handle 0.7V (most LODs are 0.5V).

The Competition – The very popular battery powered FiiO E7 overloads at 1.2V vs 1.8V for the O2. The equally popular AC powered FiiO E9 desktop amp overloads at 2.1V vs 2.8V for the O2. The Mini3 has about 6dB less overall headroom than the O2 with both running on battery power. The Mini3 also lacks a gain switch making it less flexible.

The Math – The maximum gain on AC is 7 / Vin and on battery it’s 4.5 / Vin. So with a 0.5V iPOD LOD cable it’s 7/0.5 = 14X on AC and 4.5/0.5 = 9X on battery.

Home Sources – The Redbook standard for home digital audio gear is 2V. Anything without a volume control above 2.8V would be extremely rare. But if you have such a device, just clip two resistor leads and the O2 will drop to 1X for one of the two gain settings. Or you can optimize the gain. See: All About Gain

Portable Sources – Nearly all battery powered players have a maximum output of 0.5 – 1.0V and nearly all USB powered portable DACs are 1.4V or less. All of these are 100% compatible with the O2 without changing anything.

Excess Gain – Some have claimed the O2 may not have enough “excess gain”—that is extra volume control range to “boost” improperly recorded/ripped music. Apple designed only a few dB of excess gain into their iPods because more would have been a serious compromise for 95% of the music people listen to. Too much excess gain is a problem because it renders a large portion of the volume control’s range unusable with most music. There are other negative side effects as well. Plus anyone using a PC or laptop as their source can add extra gain on the PC side to compensate for oddball recordings. They can also normalize any “quiet” tracks. With any headphones that work with the Mini3, for example, the O2 can provide at least 6 dB of excess gain even when operating on battery power. 6 dB of excess gain is plenty for more applications. With more sensitive headphones it can provide even more. See: All About Gain

WHAT ABOUT A DESKTOP ONLY VERSION? It’s possible to use the O2 board in a variety of ways in a desktop amp. This is discussed in the Enclosure Options and Circuit Board Construction sections. I’m also planning a different amp, based on the O2 design principals, for desktop-only use with the following possible benefits:

Easier to construct as a desktop amp with everything self contained on the PC board rather than having to panel mount jacks, the volume control, etc. and use point-to-point wiring.

Perhaps a few other enhancements as well such as an improved power supply

WILL THE DESKTOP AMP COST MORE? The desktop amp will require two machined panels instead of one adding about $20 and the other upgrades will add another $10 – $20 but you also save $11 as you don’t need the batteries. So if you buy both panels it might be around $30 more.

WILL THE DESKTOP AMP SOUND BETTER? It’s hard to say. The desktop amp will have even more gain flexibility than the O2 and there can be benefits to using a 1/4” headphone jack and RCA input jacks. The power supply may also have lower noise.

WHEN WILL THE DESKTOP BOARD BE AVAILABLE? Hopefully in the first quarter of 2012.

WHICH ONE SHOULD I CHOOSE? There’s a lot to be said for a portable amp. I’m using my O2 on battery power far more than I would have guessed. Do you want a BMW coupe or convertible? Both are great cars with their own benefits and disadvantages. There will be more details on the desktop version in the next several weeks.

BOTTOM LINE: I’ve tried to raise the price/performance bar for headphone amps with the O2. I’ve provided detailed measurements and the theory behind it. I hope the O2 encourages others to make, and publish, more accurate and complete measurements. I also hope it helps people smile more and worry less when listening to their music!

August 18, 2011

INTRO: In last week’s Op Amps: Myths & Facts article I covered the background for op amps, why they’re important, and some common audiophile op amp myths. This is the follow up with more technical information, a summary of many popular op amps, and some of the measurements I made.

THIS STUFF MATTERS: A lot of people play around with “op amp rolling” but they mostly lack the ability to measure the results. A lot of op amps get “upgraded” to much more expensive parts usually with zero real world benefit. Worse, these usually faster parts can easily be unstable in a circuit designed for a much slower part. Some of the differences heard could be high frequency instability. But when someone “upgrades” to an expensive op amp with high expectations many assume the different sound must be better sound. This article is all about measuring the results and putting some facts around the differences between op amps in a typical audio application.

REALITY CHECK: This isn’t a comprehensive comparison. It’s mostly a side story to my research and development of the O2 Headphone Amp. So please don’t expect a technical paper worthy of the Audio Engineering Society. A complete op amp survey is at least several full weeks worth of work. For an example of a more ambitious effort, look up Samuel Groner’s “Operational Amplifier Distortion” published in 2009. It’s a 436 page monumental effort in the form of a huge 35 MB PDF (which is why I’m not linking to it here). I’ve also referenced the work of Douglas Self who has run many op amps through an Audio Precision SYS-2700 series audio analyzer in various configurations.

LIMITED SCOPE: The O2 headphone amp uses DIY-friendly DIP8 dual op amps. It also requires op amps that can operate comfortably at +/- 12 volts (24 volts total). This rules out single devices, surface mount packages, and lower voltage op amps. While I did adjust the compensation and feedback impedance with some op amps, ultimately, the measurements are all in the same circuit configuration unless otherwise noted.

MOUSER’s SELECTION: One of the design goals of the O2 was to purchase as many parts as possible from a single vendor and, for some good reasons, that turned out to be Mouser Electronics. Mouser offers Ti, Burr Brown, JRC/NJR, On Semi and others, but they don’t offer National, Analog Devices or Linear Technology. So most of my evaluation was limited to what Mouser stocks. But I did test a couple parts from National and Analog Devices.

BLIND TESTING CHALLENGE: For those of you who believe there’s more to op amp sound than just the measurements, please read the Op Amp Myths article if you haven’t already. And if you’re still a skeptic, let’s schedule your YouTube video appearance with an ABX comparator, the op amp of your choice, and a $0.60 NE5532 using two O2 boards and the op amps in the gain stage. If you can tell them apart, I’ll give $500 to the charity of your choice. If you can’t tell them apart, you donate $500 to the charity of my choice. The challenging op amp’s measurements must at least meet the minimum criteria outlined in the O2 Design Principals. The test would be administered by an independent third party (I won’t even be present). Win or lose the video goes on YouTube and the story is published on this blog. And if you think the O2 is the weak link, please see: O2 Open Challenge

BOTTOM LINE: For those wanting to skip the Tech Section, the conclusions can be summed up as follows:

At gains less than 4X nothing overall could beat the $0.39 NJM2068 in the O2’s gain stage. This is especially true if you’re concerned about power consumption for battery operation.

At gains of 5X and higher nothing could beat the $0.65 NE5532 except by a few dB in noise performance.

The older TL072, despite being used in thousands of audio devices, is way out of its league against newer op amps. It’s no longer even in the game in a low noise application like the O2.

For the output stage, the NJM4556 was untouchable among compatible DIP8 dual op amps.

TECH SECTION

CONSTRAINTS: The testing was mostly limited to op amps suitable for the gain stage of the O2. See the O2 Design Process article for more.

APPLICATIONS: Most of this article is written from the perspective of using op amps for headphone and line level audio applications. I’m excluding high gain applications like microphone preamps, phono preamps, instrumentation equipment, etc. Used in high gain critical applications the rules can be different.

OP AMP SPECS: When you start “screening” op amps for a project you have to look at packages (DIP, SOIC, etc.) and decide if you want a single, dual, or quad part. Beyond that it mostly comes down to the specs and here are the highlights:

Intended Application – The semiconductor companies actually do know what they’re doing. They know what parameters are most important for audio applications and most design op amps with audio specifically in mind. It’s best to use those parts designed for audio, not those optimized for say video use. This seems obvious enough but several audio designers break this rule like the OPA690 in the AMB Mini3 which isn’t even specified at audio frequencies and has rather horrible specs for audio. Anyone who thinks they know more about op amp applications than the guys designing the op amps is very likely mistaken. It’s like putting tractor tires on your sports car thinking they’ll improve the traction. Stick to the tires made for sports cars.

Operating Voltage – The O2 op amps need at least a 24 volt (+/- 12) power supply. This rules out a lot of the newest parts that are designed for low voltage portable use. Also note some op amps are optimized for single supply use and, while they’ll usually work with dual supplies, they may not be an optimal choice.

Unity Gain Stable – There are some “un-compensated” op amps available that are not unity gain stable. They specify the minimum gain they can be used at. These op amps are generally best left to those who understand the complex math involved with calculating things like phase margin and who have the test equipment to verify the result (RMAA and low bandwidth scopes don’t qualify). Otherwise, stick with Unity Gain Stable compensated op amps.

Package – The O2 requires a dual op amp in a DIP8 package. Op amps also come in single and quad versions, various surface mount packages, round metal TO99 “cans”, and some high power packages for higher thermal dissipation.

Single or Dual – The crosstalk between sections of dual op amps designed for audio use is typically better than –105 dB. That’s vastly better than the rest of a headphone amp can manage so it’s not an issue. There’s no need to use a single op amp for each channel. The 3 contact plug on almost every pair of headphones creates more crosstalk all by itself. Some claim thermal coupling between channels is a problem in dual parts, but the power consumption of a gain stage is usually dominated by the quiescent current which is relatively constant. So there won’t be significant thermal variations on the die due to the music signal. A dual op amp also has the advantage of providing better matching between the channels. So for cost savings, and to save board space, the choice is obvious—use a dual op amp.

FET or Bipolar – You can broadly divide audio op amps into these two categories based on the type of transistors used in their input stage—FET or bipolar. Some claim various types of FET (JFET, Bi-FET, Di-FET, etc.) inputs “sound better” but they’re usually more noisy in real world use. The main reason to use FET inputs is for their superior DC performance (especially bias current) which is mostly useless in an audio application. The one exception being, if you’re going to hang a volume control right off the input, a FET op amp will generally put less DC through the volume control wiper. This helps reduce the “rustling” noise you get when you adjust the volume. It also can allow larger resistor values without excessive DC offset. But the O2 solves these issues in other ways so FETs are generally more of a liability if accuracy is the goal. The ultra high-end FET devices, like the Di-Fet OPA627, are an exception to the rule but the FET OPA2134 is not. It performs worse in some ways than the much cheaper bipolar 5532.

Quiescent Current – In a battery powered device the quiescent (idle) current is a critical spec. Most of this current is for biasing the internal transistors. Some op amps need as much as 9 ma per amp section or more. While low power op amps typically have poor drive capability, higher noise, lower GBW, and poor slew rate. There are significant trade-offs between power and performance.

Gain BandWidth Product (GBW/GBWP/GBP) – The GBW helps indicate how much feedback will be available at higher frequencies and higher gains to correct distortion. But higher GBW usually means more power drain and possibly less stability. For low distortion at 20 Khz it should be at least 5 Mhz for G=1. Some manufactures specify the Unity Gain frequency instead which is the same as GBW with G=1. To compare GBW between different op amps it needs to be specified at the same gain—i.e. G=1, G=10, etc. Most manufactures provide a graph as well.

Open Loop Voltage Gain (myth busted) – The open loop gain combines with the GBW above to determine how much feedback (NFB) is available across the power band. Generally, the higher the better and it should be at least 90 dB worst case. The best audio op amps are around 140 dB typical and it can make a big difference in the performance—especially at higher gains. Nearly all the audiophile myths about too much feedback have been proven false by lots of research—some of it relatively recent. Bruno Putzeys (a truly brilliant guy), in particular, has put most of the remaining persistent myths to their overdue death. May they rest in peace (but I’m sure they won’t). More on this below.

Slew Rate – Here’s another huge myth: The faster the slew rate the better. But the fastest “slew” you can get from a 16/44 digital recording has a period of about 22 uS. So the O2, worst case, needs to slew 20 volts in 22 uS or a slew rate of 0.9 V/uS. And even considering those rare 24/96 recordings, the requirement is still 1.8 V/uS. SACD or 24/192, in theory, could require about 3.6 V/uS but, in practice, never will as that would be a 0 dBFS signal close to 100 Khz. Such a signal would never make it through the recording chain, and if it did, would fry tweeters and cause bats to fly into the side of your house. The peak levels in music generally fall as the frequency rises. This is especially true at ultrasonic frequencies. If you think I’m wrong, please reference a recording than can challenge even a 1.5 V/uS headphone amp. The industry rule of thumb, even on the recording side of the signal chain, is 0.2 V/uS per volt RMS of maximum output. I’ve doubled that just to be ultra conservative so 7*0.4 = 2.8 V/uS for the O2’s requirement. As I have explained elsewhere,slew rates well in excess of what’s needed come at a price. Somewhere in the range of 2.5 – 5 V/uS is ideal for a high output headphone amp like the O2.

Feedback & Speed Myths – First of all, some audiophiles are under the mistaken impression that negative feedback (NFB) is mostly a bad thing. I just read, in an audiophile magazine, that NFB can only work correctly if an audio amp has zero delay. And they claimed such amplifiers don’t exist so the next best thing is either zero feedback or an extremely fast amplifier. The amp being discussed was touted as good for “250 Mhz” (yeah right!) resulting in “essentially zero delay”. This is a huge pile of false technobabble. The whole idea of “by the time a feedback loop corrects an error, it’s too late” is just layperson nonsense. Look up what Bruno Putzeys and Doug Self have written on the topic if you want the real truth. To use a layperson’s analogy, nearly any DC motor driving a turntable is controlled with a feedback loop. The turntable has rather massive delays with respect to current inputs to the motor vs speed due to the mass of the platter. The feedback pundits argue by the time the feedback detects an error, the speed is already wrong and can’t be corrected due to the delays. But, somehow against all those supposed audiophile gurus, the speed is rock steady and the feedback works perfectly. The same is true in an amplifier. The only speed issue that matters is the amp should never be slew rate limited. As long as that condition is satisfied, there’s no such thing as “Transient Intermodulation Distortion” or other such feedback related problems. Those are all dinosaurs from the era before fast power transistors were available for high power amps. It’s a non-issue at headphone voltages and below even using $0.39 parts like the NJM2068. Really.

CMRR – Common Mode Rejection Ratio is the ability of an op amp to reject input signals that are in-phase with respect to ground. This matters most when using an op amp in differential mode but it can help lower distortion in many single-ended circuits. For example, with the popular voltage follower op amp “buffer” shown to the right, the distortion will rise with input impedance in proportion to the op amp’s CMRR. In other words, if you stick a volume control in front of a buffer, and turn down the volume, an op amp with better CMRR may well outperform one with lesser CMRR. The close quarters inside a small portable amp results in more electromagnetic coupling of high current signals throughout the circuitry. Here again, higher CMRR helps the critical gain stage reject what otherwise might become distortion. For this application at least 80 dB worst case and 100+ dB typical is desirable. It’s also worth noting CMRR is often asymmetrical which is only sometimes disclosed on datasheets

PSRR – Power Supply Rejection Ratio is the ability of an op amp to reject common mode signals on the power supply rails such as ripple and distorted versions of the music signal. The grain of truth behind the common audiophile desire for massive overkill power supplies is they can make a difference but only when the audio circuit has insufficient PSRR. And a lot of audiophile designs have poor PSRR as they use discrete circuits or op amps not designed for audio. The OPA690 in the Mini3, for example, has poor PSRR for audio use. But if you simply use op amps with excellent PSRR, any variations on the power supply end up well below the noise floor of the entire design even with an inexpensive power supply. It’s a much more elegant and rational solution than a power supply with 50 times the required current capability. Like CMRR, PSRR can by asymmetrical.

Turn On/Off Characteristics – Unless there’s an output relay to protect the headphones it’s important the op amp be reasonably well behaved on power up and power down. Not all are. And it’s often not covered in the datasheets so you just have to test it with a real scope at a slow sweep speed (most AC coupled soundcard scopes and RMAA won’t work). Don’t try to subjectively judge the “thump” in a pair of headphones as it’s hard to hear large amounts of DC. And a DMM is nearly useless due to their very slow update rate (they’ll miss the true peak of the transient by a mile).

Drive Capability – To keep Johnson noise low, the op amp should have low distortion into a 1K load. A lot of op amps fail here—especially low power versions. This can be hard to figure out from many datasheets. Some of the better audio op amps are specified into 600 ohms. If you only see loads of 5K or higher, the part probably isn’t suitable for high-end audio. Ideally look for a THD graph at 2K or less with some gain.

Low THD – This is also often poorly specified. And if it’s not specified at all, or given as x dB for specific harmonics, you’re probably looking at the wrong op amp for an audio application. Ideally you want to know the THD at various gains, versus frequency, into various loads. In practice, this often requires testing with an audio analyzer. If the entire amp is to meet a 0.01% THD goal, the gain stage needs to be well under that.

Low Noise – Op amp noise is modeled two ways--current noise and voltage noise. The O2 is a low impedance application and the current noise will likely dominate but both have to be considered along with the Johnson noise. The math isn’t trivial. You can’t just compare the raw datasheet numbers and know how the amp will really perform in a specific application unless both the voltage noise and current noise are lower than another op amp. And even then Johnson noise could still dominate so both would perform roughly the same. And, in reality, the voltage and current specs tend to be mutually exclusive—i.e. amps with the lowest voltage noise usually have higher current noise and vice versa.

Phase Reversal – Some op amps, when their inputs are overloaded, violently slam into the opposite supply rail. And sometimes that nasty fact isn’t even on the datasheet or is buried in a foot note somewhere. Phase reversal is unacceptable in this application.

Rail-to-Rail – This isn’t required for the O2, but would be nice—especially when running from batteries. The problem is most rail-to-rail audio op amps are newer designs and often only come in surface mount packages and/or can’t handle a 24 volt supply. And the few that exist and meet the other requirements tend to be expensive. Even non-rail-to-rail op amps vary widely in how close they can get to the rails with various loads and operating voltages. The datasheets usually have a graph or two showing voltage swing versus load resistance and/or current output. It’s rarely worth using an op amp that’s otherwise poorly suited to high quality audio just because it’s rail-to-rail.

DC Offset Voltage & Bias Current – This usually isn’t an issue in an AC coupled application like the O2 but can very be a serious issue in a DC coupled application not using a servo or a high-pass filter in the feedback loop. The higher the gain the more it can matter.

Short Circuit Protection – Any op amp driving an external output (line outs, headphone jacks, etc.) should have short circuit protection unless you can add enough series resistance without compromising the performance (which is often impossible). Beware even with short circuit protection it might be short term if the device does not also have thermal protection. The datasheets are usually fairly clear on this. There’s also the chance of exceeding the Safe Operating Area (SOA) of the output stage at higher supply voltages. Some designs may require some series output resistance to meet all these requirements. There can be trade offs involved. Enclosing short circuit protection in the feedback loop, as done in the AMB Mini3, can create stability issues and increase distortion.

Equivalent Schematics – Some manufactures publish so called “equivalent schematics” for their op amps. The implication is it’s a fairly accurate representation of how they actually made the part. In reality, however, it’s often not as they don’t want to help their competitors out too much. The schematics have become rather generic so as not to give too much away. So don’t try to take them literally. There’s often a lot more going on.

Unique Advantages – For those who prefer discrete designs over op amps, I refer you to Q9 in the small snippet to the above right. Op amp designers are free to optimize every single transistor in the device including creating things you can’t buy off the shelf—like transistors with 3 collectors. This, combined with the inherent matching of transistors made on a single die, and near perfect thermal tracking, provides a bag of tricks no discrete designer can match. Sure there are advantages both ways, but the op amp guys have an awful lot at their disposal—particularly compared to some guy working in his basement.

COMPENSATION: Unity gain stable op amps, as being evaluated here, use dominant pole compensation for stability. This is to make sure that the amp does not oscillate when feedback is applied. In a low gain application, the amp might still show ringing on square waves due to the phase margin becoming small. This is usually compensated for by adding the correct value of capacitance in parallel with the feedback resistors from output to the negative input. These capacitors cause a zero and a higher frequency pole in the feedback loop response effectively cancelling one of the amp's own non-dominant high frequency poles (by placing a zero near it) and replacing it with a new one at a higher frequency. This results in an increased phase margin, improved stability, and improved transient response. It’s important to note this compensation isn’t “one size fits all”. It’s specific to many variables including the gain, the op amp’s own loop response, the impedances and reactive components in the circuit, etc. Like many things in audio it can be a delicate compromise say in an output stage with unknown external loads, or in a gain stage with two different gain options.

THE OP AMPS: Below I’ve sorted some of the more popular op amps into several categories. The ones marked with an asterisk are op amps I’ve tested in the last few months and at least some data is shown for most of them in the next section.

THE ELITE FEW: Designer op amps are amazingly like hyper-expensive supercars. They’re vastly more expensive than their more mainstream counterparts, relatively high strung, and very high maintenance. While they may show their true colors in critical applications, like say inside a $20,000 audio analyzer, it’s hard to imagine where they offer any real-world benefit inside a headphone amp. It’s analogous to taking your half a million dollar Ferrari down to the Quick-E-Mart for a corndog. The only reason not to take a perfectly competent, and more comfortable, BMW instead is so you can be seen in your Ferrari. It’s not like the Ferrari will get you through urban traffic any quicker and you’re more likely to mow down some kid on a bicycle because of the poor visibility out of that low slung bodywork. It turns out the following op amps find their way into headphone products for exactly the same reason—to be seen. It has very little to do with better real world performance or a genuine difference in sound quality. Here are some of the elite:

OPA627 – The Burr Brown/TI OPA627 might be the Bugatti Veyron of op amps as both offer some of the best performance numbers around. But, just like it’s pretty much impossible to explore the Veyron’s full performance anywhere but on a race track, the same is true with the OPA627. Ferdinand Piëch was playing corporate one-upmanship with the Veyron, and Burr Brown did the same with the OPA627. Basically it’s the op amp of choice for those who keep their towels on in the gym locker room. In exchange for your $24, TI gives you a stereo pair of op amps so quick they can easily go sideways. Unlike the Veyron, there’s no built-in stability control—that’s up to the designer to get right. With a slew rate of 55 V/uS, the OPA627 demands very careful attention to PC board layout, power supply bypassing, parasitic inductances and capacitances, and reactive loads on the output. Basically if you’re only going to drive on public roads in traffic--i.e. headphone audio applications--it’s expensive overkill and more trouble than it’s worth.

AD797 – The Analog Devices AD797 is probably the OPA627’s closest competition. There’s no question it’s a good part, but it’s also over $10 for a single op amp per package. If anything, due to it’s more rational speed (slew rate = 20 V/uS), and better CMRR/PSRR at power supply ripple frequencies, it’s probably the better part for audio applications. But this game is all about bragging rights. So, sadly, it often gets pushed aside in favor of the more trendy and excessive OPA627.

OPA604 – The Burr Brown/TI OPA604 is also designed for audio use and, unlike the 627 above, even comes in a dual version. But I’ve never bothered with it after Doug Self extensively tested it and concluded: “it is not very clear under what circumstances this op-amp would be a good choice”. He found the much less expensive 5532 outperformed it in most every way.

*AD8610 – Analog Devices lists “high performance audio” among the applications for the AD8610/AD8620 and, at around $10 each for the single part and $17 for the dual part, they aren’t cheap either. And, like the choices above, they’re overkill for headphone amp applications. I tested this part in the QRV09 and, while it’s very respectable, I suspect the NE5532 would match it to the limits of most audio analyzers.

HIGH PERFORMANCE: If the above are the ultra expensive supercar elite, these are the more attainable high performance cars. They’re still expensive and demand extra maintenance but they’re more sanely priced and closer to offering performance you can actually use now and then:

*LM4562/LME49860/LME49720 – National had a dedicated team of high-end audio engineers and they turned out some really great parts. The most popular are these op amps which have found their way into a lot of high-end audiophile blessed gear. The LM4562 and its siblings, in any headphone or DAC application I can imagine, should match any of the elite parts above. That’s objectively on an audio analyzer using the conventional suite of audio tests and subjectively in blind listening tests. The LM4562 was the first op amp to unseat the NE5532 as Doug Self’s overall benchmark. The LM4562 is electrically identical to the LME49720 but cheaper ($3) and comes in a DIY friendly DIP8 package. Samuel Groner did raise some common mode concerns with the LM4562 depending on how it’s configured. Doug Self, who also tested it in some similar configurations, seems less concerned.

*OPA2134/OPA134 – The Burr Brown dual FET OPA2134, and the OPA134 single version, are the BMW 325 sedan of audiophile op amps. You see lots of them around, they’re not exactly cheap, and they offer better than decent performance. I’ve included this part in my comparison mainly because it’s so popular. They’re $3 – $4 each for the dual part. Personally, for the price, I’d much rather have the LM4562 which outperforms it overall. I had to laugh when I read Doug Self’s critique of this part. Commenting on Burr Brown’s claims of “superior sound quality” Self said, “regrettably, but not surprisingly, no evidence is given to back up this assertion.”

*OPA2227/OPA227 – This OPA2227 is a more expensive Burr Brown bipolar op amp that offers lower noise specs and some other improvements over the 2134 above. This part is considered an upgrade over the popular OP-27 and LT1007 so I’ve included it in my tests. It has lower noise than the LM4562 but little else to recommend it in typical headphone/DAC applications.

OP275 – The Analog Devices OP275 part is another part Doug Self rated a solid FAIL. His summary: “it is probably best avoided.”

Linear Technology – Linear Technology makes lots of op amps with typically each having an impressive spec or two but they don’t seem to have the sort of home run all-rounders to match something like the LM4562. Samuel Groner tested several LT parts with mixed results. He was rather unimpressed with the LT1007 for example as it had obvious crossover distortion and other flaws.

GIANT KILLER: The following op amp deserves its own category:

*NE5532 – The NE5532, as mentioned in the last article, was originally developed by Signetics mainly for audio use. At the time it, for audio applications, it was like comparing a modern day Ferrari to an old Volkswagen Beetle. It really wasn’t until the LM4562 (see above) came along there was much reason to use anything else for most consumer audio applications—especially if you exclude microphone and phono preamps. There are op amps with better DC specs, faster settling times, higher slew rates, lower offsets, etc. But none of those improvements help 99% of audio gear perform any better. And those “partly better” parts are often worse in more critical areas. The 5532 is the corner stone of nearly all high-end professional audio gear which means most of your music has already been well steeped in 5532 goodness. If you dislike the “sound” of 5532 that means you must also dislike most of the music available.

*NJM2068 – You’ll find the NJM2068 in a fair amount of pro audio gear—some of it with big names on the outside. I haven’t seen it as much in consumer electronics but it does show up now and then. It has significantly better noise performance than even the NE5532, LM4562 and far better than the OPA2134. It’s $0.39 and requires about a third less power than most of its competitors. It can’t drive as low of impedance loads, and can’t quite match the 5532’s high frequency distortion at higher gains, but it comes so close in most applications it’s essentially a tie.

*NJM4562 – How about a $3 LM4562 (see above) for $1? That’s pretty much what you get with the NJM4562. My measurements show the two to be very similar with the NJM version even needing less quiescent current. It has slightly more noise than the 2068 above but can better drive low impedance loads, with lower high frequency distortion, at higher gains. For gains < 4X into “easy” (non-reactive) loads of 2K or higher, the NJM2068 is the better part. If you’d rather have the Audi A3 over the very similar but much cheaper Volkswagen GTI, get the National part, otherwise save some money with the NJM4562.

HIGH CURRENT: For driving low impedance and/or significantly reactive loads, there are not many choices in dual DIP8 packages that work comfortably at +/- 12 Volts (24 volts total). The short list:

*NJM4556 – If you need lots of drive capability the NJM4556 can beat every dual DIP8 op amp I know of. This part is discussed more in the O2 Design Process article.

*RC4580/NJM4580 – The 4580 isn’t a bad part but, in my tests, it’s inferior to the NJM4556 in pretty much every way. Most significantly it gives up up at around 50 mA while the 4556 can manage double that. And they’re only about $0.20 cheaper.

DON’T BOTHER: While the following parts had their glory days, they’re long gone. They offer no advantage over the above parts and are often much worse:

*TL071/TL072 – These were mainstream audio parts and used in lots of pro gear before the 5532 came along. For all those speed freaks, it’s worth noting they’re rated only slightly slower than the elite chips at 15 V/uS. They were also among the first reasonably priced FET op amps that were useful for high quality audio. But they suffer from very limited output current, they can’t get very close to the rails, have higher distortion, and they’re relatively noisy. Their lower power cousins the TL061/TL062, and the lower spec TL081/TL082 are even worse.

LM833 – This was one a National’s early attempts at a high performance audio op amp like the 5532 but they failed to beat the 5532.

4560 – The 4560 has been eclipsed by the NJM2068, 5532, 4556, etc.

4558 – Same as above

Most Any CMOS Op Amp – There are lots of CMOS op amps (such as the TI TLC and TLV series) with various marketing claims being made. They’re great for lots of non-audio applications. But I’ve not seen any that can even come close to the $0.39 NJM2068 for serious audio performance into similar loads, etc. Unless you have a specific reason (such as a strict low power/low voltage requirement) to use one there are generally better choices

MEASUREMENTS

WHICH PARTS: To avoid spending 100+ hours on this, I’m only publishing the cream of the crop. Overall I tested nearly two dozen op amps. But it quickly became apparent that many were either obviously inferior, or in this application, approached the limits of the dScope which means they were way past being “good enough”. So I then narrowed the choices down further based on price, popularity, and power consumption. For the output stage it was no contest, the NJM4556 killed everything else. For the gain stage the final list worked out to:

NJM2068 – Cheap and Cheerful

NE5532 – The reasonably priced objective benchmark

OPA2134 – A mainstream audiophile favorite

OPA2227 – Pushing the high-end of the budget with excellent specs

OPA2277 – For the low power O2 (long battery life version)

LM4562 – An audiophile favorite (NJM4562 stocked at Mouser)

TL072 – A former audio benchmark to put the above in perspective

THE CIRCUIT: The O2’s gain stage at the right was used with various resistor values from R16 and C19 to ground to achieve 2.5X, 3X and 7X gain (as noted in the tests). All the Op Amps were measured with the same 1.5K feedback resistor and 220 pF dominant pole capacitor. Pin 7 was connected to a 10K pot yielding about a 1.3K net load. For the full schematic, and more details, see the Circuit Description section of the O2 Details Article.

ALL THE SAME: Here’s a list of measurements that were essentially unchanged by any of the op amps and why:

Frequency Response – The 220 pF feedback cap determines the upper –3 dB point (~250 Khz) with all the op amps and the low frequency limit of just the gain stage was down to DC. There were no measureable differences. The only thing that would cause a problem would be high frequency instability might cause a rising HF response. But I checked for that where applicable.

Slew Rate – All were over the 2.8 V/uS requirement for the O2. Put another way, all were over the requirement for any dynamic headphone amp.

Square Wave/Stability – Stability issues showed up in the THD vs Frequency sweeps as added high frequency distortion or worse. So I did check the square wave response for any op amps showing unexpected behavior. None of those op amps are shown below (i.e. everything measured below was stable as far as I know).

Phase – This is set by the 220 pF feedback compensation capacitor to less than 1 degree error at 10 Khz with all the op amps.

Crosstalk – The internal crosstalk in any of the parts I tested was lower (better than –100 dB) than the rest of the circuit (such as the volume control and headphone jack) so it wasn’t a significant factor.

Clipping Performance – I probably should have verified the clipping behavior on a scope, but I didn’t. The good news is the THD vs Output sweep tends to show red flags for any weird clipping behavior and all op amps were driven to clipping in that test.

SMPTE & CCIF IMD – IMD testing might have revealed some minor differences. But the few I checked were so similar I didn’t bother going further. I suspect only the TL072 would have stood out.

DC Offset – It doesn’t matter in the O2 because the gain stage is AC coupled to the output stage so it wasn’t worth measuring. The datasheets should yield accurate numbers for those interested.

Distortion Magnifier – It would be interesting to use a distortion magnifier to increase the resolution of the dScope distortion measurements by an order of magnitude to reveal more differences. I have one, but it’s a lot more work to use than the dScope’s native hardware. I’m really not that concerned with distortion in the “triple zero” range given that it’s at least ten times below the most conservative estimates as to what’s audible (0.01%). The dScope reads as low as 0.0005% on its own. I’m not trying to count DNA strings in an electron microscope. I just want to listen to music!

dSCOPE RESIDUAL THD+N: Several things are important to know about the dScope. First these tests are using its Continuous Time hardware analyzer which computes the THD+N in real time. This dramatically speeds up sweeps with 100 points for each pass but it also limits the accuracy and ultimate noise floor compared to using FFT post-processing. The white line in the two graphs below is the residual noise floor of the dScope in loopback and the THD+N is as low as 0.0005% which is plenty low enough for these measurements. But, op amps have become so good they can easily approach or even better that. Keep in mind that 0.0005% is the combined distortion of the signal generator, output amplifier, all the balanced isolated input circuitry (which can handle anything from a few microvolts up to 200 volts RMS!), and the 24/192 A/D in the dScope. So there’s a lot more going on in the dScope than just a single op amp so 0.0005%, viewed in that light, is rather impressive.

DROOPING HF DISTORTION: It’s somewhat debated, but the generally accepted practice limits THD+N measurements to the audio band. This has especially been true in the “digital age” where there can be significant out of band noise from digital circuitry, switching power supplies, etc. If you include the out of band stuff, the entire noise floor rises and masks the true audio performance below 20 Khz. So I’ve cut all but one of the sweeps off at 22 Khz which is the default standard in the dScope. The distortion drops as the harmonics fall above this frequency. For example the third harmonic of 8 Khz is 24 Khz and would be rejected by the analyzer.

JAGGED LINES & LEGENDS: The jagged bits in the traces are mostly the input circuitry auto-ranging to keep the signal within 2 dB of the dScope ADC’s 0 dBFS point. This maximizes the dynamic range of the analyzer but causes some “steps” in the sweep when the relays switch different attenuators into the circuit. Audio Precision analyzers do the same thing (unless you average it out with lots of smoothing). Finally the “A” in the legend names is “Channel A” of the analyzer and it’s added automatically. It’s not my usual poor grammar.

THD+N vs OUTPUT: The most interesting thing about this result is how boring it is. Note: This is just the gain stage. The dScope was connected directly to the output pin of the op amp being tested (the NJM4556 output stage is not included). The default NJM2068 is shown at gains of both 2.5X (yellow) and 7X (aqua) while the others are shown at only 7X. The OPA2134 (blue) and the NE5532 (pink) get slightly closer to the rails before they clip but otherwise deliver very similar performance—all 3 parts are so close to the analyzer’s residual it’s mostly a tie. I didn’t even include the OPA2227 or LM4562 as the lines were right on top of the NE5532. The TL072 (orange), however is a different story. The TL072 clearly isn’t liking the ~1.3K load. Note the graph has been re-scaled from my usual 1% THD to only 0.1% at the top. The DC rails are +/- 11.85 V:

LOW POWER OPA2277 THD+N vs OUTPUT: Here’s the low power op amp I’m recommending for the O2 low power option (25+ hour battery life) at both gain settings compared to the standard NJM2068. While the clip points differ by about 10% the distortion performance is very similar. The OPA2277 needs only 0.8 mA per amp or 1.6 mA total vs about 4.5 mA total for the NJM2068. The big downside is it’s more than ten times the cost ($4.40). It’s the best performing low power part I found, many others like the On Semi MC33078, TL062, NJM022, etc. are seriously awful:

THD+N vs FREQUENCY: Here are the favorites plotted vs frequency also taking the output directly from the gain stage op amp under test. Again, I didn’t bother with the expensive OPA2227 or LM4562 as they just further blurred everything together with the residual of the analyzer. At 2.5X gain the NJM2068 matches everything else. At 7X gain (aqua), however, it’s not quite as happy with the ~1.3K load and starts to show the limitations of a $0.39 part. However, to put it’s 7X gain 0.0018% max THD+N in perspective, consider that’s 28 times lower than the 0.05% high frequency requirement. Even if you use the much more stringent –80 dB threshold of 0.01% it’s still more than 5 times lower. So it’s extremely unlikely to be an audible problem. And, perhaps more important, the NJM2068 uses significantly less power and has significantly less noise. The TL072, on the other hand, is obviously having a hard time as it did in the earlier sweep:

LOW POWER OPA2277 THD+N vs FREQUENCY: Here’s the recommended low power op amp versus the default NJM2068. At 2.5X gain the OPA2277 is still under 0.002% while at 7X gain it hits 0.0045% which is still well under the 0.05% high frequency goal and even comfortably under 0.01%. This is a worthwhile trade off for needing only about 1/3 the power:

OTHER TESTS: Here the dScope bandwidth extends to 44 Khz testing the older V1.0 O2 board at 3X gain.The measurement is at the headphone output into 600 ohms at full volume. The important thing to note is even the relatively high-end LM4562 and OPA2227 don’t make any difference:

dSCOPE RESIDUAL NOISE: Here’s the noise floor of the dScope in dBV (the unit dBV is always referenced to 1 Volt RMS) in loop back with the output of the generator muted and set for a 25 ohm source impedance. To reference these numbers to the full output of the O2 (7 V RMS) you would add 17 dB so the A-Weighted number would be 137 dBr (ref 7V). Also note the noise floor is well below –150 dB except close to 20 hz (which is actually an artifact). The total sum of the noise out to 22 Khz is how the –116.8 dB number is calculated:

NJM2068DD NOISE: The default op amp is impressively quiet even at 7X gain as measured here—especially for $0.39! Being a bipolar input device designed for audio and low noise I would expect decent performance but this is borderline stunning matching even the nearly $5 OPA2227 below:

NE5532 NOISE: The professional audio favorite, the NE5532, is about 3 dB worse at 7X gain than the less expensive and less current hungry NJM2068 above but still very quiet. I have heard some versions of the 5532 are a few dB better than others. This was a TI NE5532AP:

LOW POWER OPA2277 NOISE: This is the recommended op amp for the low power version of the O2. As is often the case with lower power op amps, despite it’s $4+ price, it has roughly 6 dB more noise than the $0.39 NJM2068 but it’s not too bad all things considered and matches the popular OPA2134. Most of the other low power parts I tested were much worse:

OPA134/OPA2134 NOISE: Op amps with FET inputs are typically noisier in many audio applications. And despite the Burr Brown OPA134/2134 being marketed as “low noise” it’s has nearly 3 dB more noise than the much cheaper NE5532 and nearly 6 dB more noise than the cheaper still NJM part. The –130 dB spike at 60 hz might be related the high impedance FET inputs or it could just be changes in the AC field around my test bench but it’s not significantly affecting the numbers. You can plainly see the entire noise floor is about 3 dB higher than the graph above:

OPA227/2227 NOISE: This is the most expensive op amp at nearly $5 and, for 12 times the cost of the NJM2068, it only manages to match its noise performance! This is a bipolar input device specifically designed for low noise applications in professional audio equipment. In the O2 this would be a complete waste of nearly $5:

LM4562 NOISE: While the National LM4562 isn’t available from Mouser, the NJM4562 is and they perform very similarly. Here’s the genuine National part and it’s noise performance turns out to to be nearly identical to the NE5532:

TL072 NOISE: The TL072’s FET inputs and ancient design really show here. It’s nearly 12 dB worse than the NJM part which is fairly huge—especially considering the TL072 is more expensive (about $0.60). The 60 hz spike is back which has me wondering if FET inputs and EMI susceptibility go together (see the FET OPA2134 above):

MEASUREMENT SUMMARY: While there are some differences, if you exclude the ancient TL072, everything else delivers very similar distortion performance, especially at more modest gains. There are perhaps more significant differences in noise performance and that’s where the NJM2068 excels. The NJM2068 might have slightly more high frequency distortion at 7X gain, but it also has 3 dB lower noise and uses 1/3 less power than the NE5532. The overwhelming conclusion is you can spend less than $1 to get performance well beyond the point of diminishing returns. Spending more doesn’t buy you anything useful in this typical application. Hopefully this article at least helps demonstrate there are not big differences between appropriate op amps in a well designed circuit. If you had a headphone amp with an NE5532, and tried to “upgrade it” to an OPA2134, you would only end up with about 3 dB more noise for your trouble.